The present invention relates to communications in computer networks. More specifically, it relates to a method for dynamically optimizing performance in a data-over-cable system.
Cable television networks such as those provided by Comcast Cable Communications, Inc., of Philadelphia, Pa., Cox Communications of Atlanta Ga., Tele-Communications, Inc., of Englewood Colo., Time-Warner Cable, of Marietta Ga., Continental Cablevision, Inc., of Boston Mass., and others provide cable television service to a large number of subscribers over a large geographical area. The cable television networks typically are interconnected by cables such as coaxial cables or a Hybrid Fiber/Coaxial (xe2x80x9cHFCxe2x80x9d) cable system. The system can also provide data services having data rates of about 10 Mega-bits-per-second (xe2x80x9cMbpsxe2x80x9d) to 30+ Mbps per channel.
The Internet, a world-wide-network of interconnected computers, provides multi-media content including audio, video, graphics and text that requires a large bandwidth for downloading and viewing. Most Internet Service Providers (xe2x80x9cISPsxe2x80x9d) allow customers to connect to the Internet via a serial telephone line from a public switched telephone network at data rates including 14,400 bps, 28,800 bps, 33,600 bps, 56,000 bps and others that are much slower than the about 10 Mbps to 30+ Mbps available on a coaxial cable or HFC cable system on a cable television network.
With the explosive growth of the Internet, many customers have desired to use the larger bandwidth of a cable television network to connect to the Internet and other computer networks. Cable modems, such as those provided by 3Com Corporation of Santa Clara, Calif., and others offer customers higher-speed connectivity to the Internet, an intranet, local area networks (xe2x80x9cLANsxe2x80x9d) and other computer networks via cable television networks. These cable modems currently support a data connection to the Internet and other computer networks via a cable television network with a data rate of up to 30+ Mbps which is a much larger data rate than can be supported by a modem used over a serial telephone line.
Background information related to cable modem systems in general is described in the Data-Over-Cable Service Interface Specifications (xe2x80x9cDOCSISxe2x80x9d)xe2x80x94Radio Frequency Interface Specifications, Interim Draft, dated Jul. 24, 1998, issued by Cable Television Laboratories, Inc. DOCSIS may be found on the World Wide Web. This document, known to persons working in the art, is incorporated by reference herein in its entirety.
The basic overall architecture of a data-over-cable system is shown in FIG. 1. The system of FIG. 1 provides a mechanism by which a computer 10 connected to a backbone network 12 (either directly or indirectly by intermediate networks) may communicate with another computer 14 via a cable television infrastructure indicated generally by reference numeral 16. The cable television infrastructure 16 includes a distribution hub or xe2x80x9chead-endxe2x80x9d 18 that is connected to the backbone network 12 via a wide area network (xe2x80x9cWANxe2x80x9d) and a switch or router 20. A cable system head-end 18 is a central location in the cable television network that is responsible for sending cable signals in the downstream direction. The head-end 18 modulates 20 digital data into analog form and supplies analog signals to a fiber network 22, which is connected to a plurality of optical/electronic (xe2x80x9cO/Exe2x80x9d) nodes 24. The O/E nodes 24 convert optical signals in the fiber network 22 to electrical signals for transmission over a coax cable network 26 to a cable modem 28 at the customer""s location. The cable modem 28 demodulates the analog signals and extracts the digital data and supplies the data to the customer premise equipment (xe2x80x9cCPExe2x80x9d) 14, which, in a typical situation, is a general purpose computer in a home environment.
The head-end 18 includes a cable modem termination system (xe2x80x9cCMTSxe2x80x9d) 30. This device provides a network side interface to a wide area network, indicated at 32, and a radio frequency (xe2x80x9cRFxe2x80x9d) interface between the cable modem termination system and the cable network in both the downstream and upstream directions, indicated at 34 and 36. The term xe2x80x9cdownstreamxe2x80x9d, as used in the present document, refers to transmission in the direction from the head-end 18 or cable modem termination system 30 to the cable modem 28 at the customer premises. The term xe2x80x9cupstreamxe2x80x9d refers to transmission in the direction from the cable modem 28 at the customer premises to the cable modem termination system 30.
For transmission in the downstream direction, the cable modem termination system 30 supplies data from the computer 10 to a modulation circuit (xe2x80x9cMODxe2x80x9d) and to a combiner 38, where the data is combined with video signals for the cable television system. The combined signals are sent to a transmission module 40 where they are imparted onto the fiber network. In the receiving direction, data from the CPE 14 is received from the fiber network at a receive module 42, sent to a splitter and filter bank 44 and sent to a demodulation circuit (xe2x80x9cDEMODxe2x80x9d) in the cable modem termination system 30. The data is processed by a network termination unit 46, sent to the switch or router 20 and routed onto the WAN for transmission to the remote computer 10.
Many cable television networks provide only uni-directional cable systems, supporting only a xe2x80x9cdownstreamxe2x80x9d cable data path. A return data path via a telephone network (i.e., a xe2x80x9ctelephony returnxe2x80x9d), such as a public switched telephone network provided by ATandT, GTE, Sprint, MCI and others, is typically used for an xe2x80x9cupstreamxe2x80x9d data path. A cable television system with an upstream connection to a telephony network is called a xe2x80x9cdata-over-cable system with telephony return.xe2x80x9d Such a return system is indicated at 48 where the cable modem 28 is also shown connected to the public switched telephone network (xe2x80x9cPSTNxe2x80x9d).
An exemplary data-over-cable system with telephony return includes customer premises equipment (e.g., a customer computer), a cable modem, a cable modem termination system, a cable television network, a public switched telephone network, a telephony remote access concentrator (xe2x80x9cTRACxe2x80x9d) 49 and a backbone data network 12 (e.g., the Internet). The cable modem termination system 30 and the telephony remote access concentrator 49 together are called a xe2x80x9ctelephony return termination system.xe2x80x9d
In a two-way cable system without telephony return, also termed a bi-directional cable system, the customer premises equipment 14 sends data packets to the cable modem 28, which sends the data packets upstream via the cable television network 22 and 26 to the cable modem termination system 30. Such a system is shown in FIG. 1. The cable modem termination system 30 sends the data packets to appropriate hosts on the data network 12. The cable modem termination system 30 sends the response data packets back to the appropriate cable modem 28.
In a bi-directional cable system, the cable modem termination system 30 can continuously collect information about the level of impairments on the upstream RF path of a cable plant, i.e., the portion of the network between the demodulation circuit in the cable modem termination system 30 and the cable modems 28. Further, a single O/E node 24 may serve multiple channels and cable modems. Measurements such as the noise floor level, and signal-to-noise ratio per cable modem transmission, can be made for the coax and fiber networks, along with the tracking of which cable modems are active during a given measurement interval.
The data-carrying performance of the upstream channels may vary with the conditions for radio frequency propagation on the cable network. Defective radio frequency interfaces may introduce sufficient noise into an upstream channel that the noise significantly impairs the ability of the channel to transport data packets from the cable modems to the cable modem termination system without error. Extraneous sources of radio frequency, such as citizen band or amateur radio broadcasts, may also infiltrate the upstream channels, interfere with the radio frequency carriers for the upstream channels, increase the packet error rate, reduce the data throughput, and generally impair the performance of the data-over-cable network.
Previous methods do not determine the source of the RF impairment. The source may not be readily apparent to the user (e.g., technician or cable system operator) if the degradation in the data transmission is intermittent or across multiple upstream channels. The impairment may even occur when the faulty cable modem is not transmitting. Additionally, the cause of the impairment may be more than just a noisy cable modem or an interfering external RF source. Other causes for degradation may include cross-talk between cable modems on upstream cable paths, physical defects in upstream cable paths, bugs in one or more cable modems"" software for time division multiplexing, or flaws in the O/E nodes in the data-over-cable system. However, by monitoring the condition of the network when different cable modems are transmitting it may be possible to detect, isolate, and ultimately remove the source of the degradation. In order for the network condition data to be useful, there must be a method of correlating impairments with upstream channels, cable modems, and impairment levels in order for the user to determine the source of the particular impairment.
It is therefore desirable to improve the monitoring of transmissions on the upstream network to determine the source of problems in the data-over-cable network. Determining the source of problems may improve the performance of the data-over-cable network.
In accordance with preferred embodiments of the present invention, some of the problems associated with detecting the source of problems in a data-over-cable system are overcome. Methods and systems for determining a source of radio frequency impairment in a data-over-cable system are provided. One aspect of the invention includes a method for determining a source of radio frequency impairment on an upstream path where the upstream path comprises an upstream channel. The method includes ascertaining a reference signal-to-noise ratio. A plurality of signal-to-noise ratios is measured during a plurality of active periods. One cable modem transmits during each active period. Each measured signal-to-noise ratio is compared to the reference signal-to-noise ratio to obtain a degradation value for the active period. It is determined whether the degradation value is greater than a threshold, and if so, parameters associated with the cable modem that is active during the active period are logged. The logged parameters and the source of radio frequency impairment is identified.
Another aspect of the invention includes a method for determining a source of radio frequency impairment on an upstream path where the upstream path comprises a plurality of upstream channels. The method includes ascertaining a plurality of reference signal-to-noise ratios. Each reference signal-to-noise ratio corresponds to an upstream channel. A plurality of all-channels-active periods is scheduled. All upstream channels are simultaneously active during each all-channels-active period and only one cable modem is transmitting on each of the active upstream channels. Degradation information for the all-channels-active periods is logged. A plurality of single-channel-active periods is scheduled. One upstream channel is active during each single-channel-active period and only one cable modem is transmitting on the active upstream channel. Degradation information for the single-channel-active periods is logged. The logged degradation information is correlated and the source of radio frequency impairment is identified.
However, the present invention is not limited to cable modems, cable modem termination systems, all-channels-active periods, single-channel-active periods, and other network devices and configurations of periods of upstream channel activity could be used. The foregoing and other features and advantages of preferred embodiments of the present invention will be more readily apparent from the following detailed description, which proceeds with references to the accompanying drawings.